A series of sulfhydryl compounds have been developed by Walter Reed Army Institute, which protect normal tissues against radiation injury. WR 2721, one drug in this series gives a dose modifying factor (i.e., ratio of radiation doses that give the same level of biological damage with the drug as without the drug) ranging from 1.2 to 2.5 depending on the normal tissue tested. This drug has been shown not to protect murine tumors from radiation. This drug is now being used in clinical trials in radiation therapy. The purpose of this work is to study the ability of this drug to protect a rapidly proliferating and slowly proliferating tissue, i.e., skin and lung respectively, in mice, and to compare this drug with other radioprotectors available from Walter Reed.